1. Field of the Invention
The invention relates generally to franking machines which are now widely, used in corporate mailrooms.
It relates more particularly to a franking machine incorporating an inkjet print head.
2. Description of the Prior Art
The inkjet printing technology is already widely used in the field of document printing and is increasingly taking over from laser printing. Inkjet printing technology is only beginning to penetrate the field of franking machines as a substitute for conventional printing using print wheels.
A constant concern of franking machine manufacturers is to offer the public franking machines at the lowest possible cost. The aim of reducing costs makes it desirable to incorporate into these machines, to the greatest possible degree, off the shelf components that are already widely used and proven in service. This applies to the print head of a franking machine using the inkjet technology.
An inkjet print head of a franking machine includes a large number of mechanical or electronic components in addition to the print cartridge which carries out the inkjet printing. This cartridge is usually in the form of a small box in which a plurality of nozzles are mounted in a row. These nozzles are fed with ink by a micropump which draws the ink from an ink reservoir. They are controlled by an electronic circuit which excites them individually to cause them to eject droplets of ink.
Inkjet print cartridges of this kind are available off the shelf for use in printers to be connected to a microcomputer. The range of such cartridges available is extremely varied. There are cartridges with 64 nozzles, 128 nozzles, and so on. The number of nozzles in the cartridge usually determines one dimension of the pattern that the latter can print. Taking a cartridge with 256 nozzles, for example, with the nozzles spaced by {fraction (1/200)} inch, this dimension of the pattern that can be printed by the cartridge is therefore 32.5 mm. In the case of a cartridge with 128 nozzles, this dimension of the pattern that can be printed is 16.25 mm for the same pixel density.
The postal marks to be printed on mail items by a franking machine must have a stamp height of 30 mm, for example. This dimension is imposed by the postal authorities. It may vary from one country to another. To print these postal marks it is possible to use a plurality of print cartridges having 128 nozzles or 64 nozzles or a single cartridge with 256 nozzles. In both cases, the inkjet print cartridge(s) is (or are) mounted in a franking machine so that the row of nozzles is substantially perpendicular to the direction in which mail items to be franked pass under the print head of the machine.
The use of a plurality of inkjet print cartridges complicates control of the nozzles. It is easier to use an inkjet print head with a row of 256 nozzles. In this case, it is clear that some of the nozzles (16 of the nozzles in the row) do not need to be excited to print any postal mark because the latter""s 30 mm dimension is less than the length of the row of nozzles.
Each nozzle of an inkjet print cartridge is in the form of two partitions made from a piezoelectric material, for example. To eject a droplet of ink from the nozzle the piezoelectric walls of the nozzle are deformed by applying an electric voltage to them. If the 256 nozzles of a cartridge of this kind are numbered from B0 through B255, the cartridge comprises 257 piezoelectric partitions numbered from 0 through 256. Since 16 nozzles do not need to be excited, partitions 240 through 256 are not normally excited by the electrical voltage. As a result the ink inside nozzles B240 through B255 is never ejected. If any nozzle is not excited for some time, the ink inside it tends to dry, which clogs the nozzle, and the dried ink in the nozzle is likely to block any movement of the piezoelectric walls of the nozzle in question. This phenomenon of immobilization of the piezoelectric walls of a nozzle propagates from nozzle to nozzle and eventually leads to general dysfunctioning of the print cartridge.
An object of the invention is to remedy this drawback.
To this end, the invention consists in a franking machine including an inkjet print head including a row of nozzles operated selectively under the control of control means to print a postal mark on a mail item passing under the print head and in which only consecutive first nozzles of the row of nozzles forming a subset of the set of available nozzles in the row are operated by the control means to print the postal mark, wherein the control means are further adapted to operate other nozzles of the row of nozzles, different than said first nozzles, when there is no mail item in front of the print head in order to purge said other nozzles in order to prevent them becoming clogged in time.
If the first nozzles are the nozzles numbered B0 through B239, the control means prevent the ink remaining in nozzles B240 through B255 drying out and preventing operation of those nozzles and, by virtue of the knock-on effect, the operation of other nozzles.
An ink recovery reservoir is provided in the machine, under the row of nozzles, to collect the ink ejected by nozzles B240 through B255.
The control means is adapted to excite nozzles B240 through B255 is a cyclic fashion, as often as necessary to prevent them clogging, a procedure which is easy to program.
Nozzles B240 through B255 are preferably excited in the time interval available between the passage of one mail item in front of the print head and the passage of a subsequent mail item in front of the print head. This time interval is more than sufficient to carry out this operation, which does not disturb the normal franking of mail items and does not affect the throughput of the machine.
In one embodiment of the invention the control means are adapted to operate different subsets of first nozzles alternately, those subsets occupying offset positions within the set of nozzles available in the row of nozzles in order to excite all the nozzles in the row of nozzles over a period of time.
This achieves substantially the same result as previously without requiring any ink recovery reservoir. This procedure also distributes wear of the print head substantially uniformly over all the nozzles.
In both embodiments a counter for each nozzle counts the number of times the nozzle is excited in order to produce a warning message or to inhibit operation of the franking machine if any of the counters exceeds a threshold value representing a level of wear of the corresponding nozzle. It is to be understood that these counters can be implemented in software or by any equivalent means. This procedure enhances the reliability of the franking machine by preventing the risk of incomplete printing of a postal mark.